1. Field of the Invention
This invention relates to a cleaning device for an electrostatic reproducing apparatus, which can reduce an extent of fog due to a failure of cleaning, prevent deterioration of photosensitive members, and provide a stable cleaning effect without suffering any influence from circumstances at all times.
2. Description of the Prior Art
In electrostatic reproducing apparatus including an electrophotographic reproducing machine, electrostatic non-impact printer, facsimile and the like, recording images are reproduced, for example, in such a manner that a reflected light or an electrical information signal obtained through scanned exposing of a document is used to form an electrostatic latent image of the document or the electrical information on a photosensitive or dielectric member, the electrostatic latent image is developed and converted into a visible image, and the visible image is transferred onto recording paper and then fixed. Referring to FIG. 1 there is shown a schematic arrangement of a electrophotographic reproducing machine in the prior art. Light from an illuminating lamp 2 is irradiated onto a document G placed on a document glass plate 1 which is moving in the direction indicated by a solid arrow. Reflected light from the document G is projected on a photosensitive member 3a of a rotary drum 3 rotating in the direction indicated and hence an electrostatic latent image of the document G is formed on the photosensitive member 3a. A charging electrode 4 adapted to uniformly charge the photosensitive member 3a, a developing device 5, a transferring electrode 6, a separation electrode 7, a charge eliminating electrode 8 and a cleaning device (fur brush type) 9 are disposed in the periphery of the rotary drum 3 in due order. The electrostatic latent image is developed by the developing device 5 to be converted into a visible image and this visible image is transferred through the transferring electrode 6 onto recording paper P which is fed with a conveyance unit 11 from a paper feeding unit 10 such as a cassette. After transferring, the recording paper P is separated from the photosensitive member 3a through the separation electrode 7 and then sent to a fixing device 13 with another conveyance unit 12, wherein it is sent out to a paper discharging tray 14 after fixing.
To permit a repeated use of the photosensitive member 3a, it is required in the apparatus as mentioned above to remove electric charges and toners which remain on the surface of the photosensitive member 3a even after transferring among the series of aforesaid electrophotographic processes. So the residual electric charges are removed by the charge eliminating electrode 8 and the residual toners are removed by the cleaning device 9, respectively.
As a cleaning device used for electrophotographic reproducing machines, there are known a fur brush type, blade type and a magnetic brush type. The fur brush type is of a method such that a fur brush made of furs of animals such as a rabbit or synthetic fibers is continuously rotated to remove toners remaining on the surface of the photosensitive member, the toners having adhered to the fur brush are shaken off by a toner shaking-off plate (the so-called flicker bar) which is disposed in contact with the fur brush, and then the shaken-off toners are suctioned under vacuum or with a fan to be recovered through a filter. The blade type is of a method such that a blade formed of an elastic synthetic resin is press-abutted onto the surface of the photosensitive member thereby to remove the residual toners. Meanwhile, the magnetic type is of a method such that a non-magnetic cylindrical sleeve including therein magnet groups, which are disposed to have different polarities alternately, is provided near the surface of the photosensitive member, the magnet groups are rotated thereby to attract and remove the toners remaining on the surface of the photosensitive member, and the toners on the sleeve surface are shaken off by a blade or the like to be recovered. In recent years, along with an advent of high-speed reproducing machines and an increase in a moving speed of the photosensitive member, the fur brush type has been adopted in such high-speed machines because the blade type can not provide a satisfactory cleaning effect. Particularly, the fur brush has additional advantageous features that it will not damage the surface of the photosensitive member, e.g., selenium drum, unlike the blade which is pressed thereon, fine solids will not adhere to the brush and lead to damage, and hence the service life of the photosensitive member can be prolonged. For that reason, there is a tendency toward a widespread use of the fur brush type cleaning device.
In the conventional cleaning device of fur brush type, a fur brush is rotated at a high speed in sliding contact with the surface of a photosensitive member thereby to brush off toners remaining on that surface, the toners having adhered to the brush fibers are shaken off by a flicker bar which is disposed in contact with the brush fibers, and then the shaken-off toners are suctioned by a fan to be recovered with a filter bank. However, the use of the filter bank and the fan requires a large space. Thus, instead of those members, an electrically biased electrode member (referred to as a recovering roller hereinafter) is rotated in contact with the brush fibers of the fur brush, so that the toners having adhered to the brush fibers are electrostatically transfered onto the recovering roller and then the transferred toners are scraped off by a toner scraping-off blade which is provided in contact with the recovering roller (for example, Japanese Patent Publication No. 20,227/74). By so doing, it becomes possible to reduce the size of the cleaning device.
In the fur brush type cleaning device using the aforesaid recovering roller, a bias voltage is applied to the recovering roller and a current flows into the photosensitive member from the recovering roller through the brush fibers. Therefore, the device may suffer the problem that the electric charges of toners on the surface of the photosensitive member are decreased, or the polarity of the electric charges of toners is changed in the case of large bias voltage. Hence, the electrostatic attraction force between the brush fibers and the toners is so reduced that it becomes hard for the toners brushed off through a friction of the brush fibers with the photosensitive member to adhere to the brush fibers. As a result, the toners brushed off by the brush fibers are liable to adhere onto the photosensitive member once again, whereby there occurs fog or a filming phenomenon in which the toners on the photosensitive member are melted with friction heat generated between the brush fibers and the photosensitive member and then adhered onto the photosensitive member in the form of a thin film. Upon this, deterioration of the photosensitive member is accelerated and a service life thereof is shortened. Moreover, among the toners adhering to the brush fibers, those toners having adhered near the brush core become hard to transfer on the recovering roller, so that the brush fibers are liable to be clogged with toners. FIG. 2 shows the relationship between the bias voltage applied to the recovering roller and the current flowing into the photosensitive member in the case where the brush fibers are made of rayon.
Further, in the fur brush type cleaning device using the aforesaid recovering roller, the toners having adhered to the brush fibers are electrostatically attracted thereto with the aid of frictional electricity on the brush fibers of the fur brush and hence, the electrostatic attraction force between the brush fibers and the toners on the surface of the photosensitive member is relatively large. Therefore, the toners having adhered to the brush fibers are not transferred onto the recovering roller completely and this may lead to the problem that the toners are melted and firmly stick to the brush fibers, the brush fibers are soiled with the toners, and an electrostatic attraction force between the brush fibers and the toners is decreased, thus not resulting in a stable cleaning effect. On the other hand, since frictional electricity on the brush fibers is liable to suffer an influence of humidity, the potential of charged electricity on the brush fibers is largely changed depending on the circumstance. For example, under the circumstance of high humidity, the potential of charged electricity on the brush fibers becomes small and a sufficient cleaning effect can not be attained.
Accordingly, there is proposed a method wherein conductive brush fibers are used to impart the potential to the brush fibers in place of insulative brush fibers. But in this case, since the state of toners having adhered to the brush fibers and the contact state of the brush fibers with the recovering roller are not always constant, the potential of the brush fibers may be changed, thus not resulting in a stable cleaning effect.
In addition, the potential of the brush fibers is increased due to friction with the photosensitive member, so that it becomes hard for the toners having adhered to the brush fibers to move onto the recovering roller, even if high voltage (e.g., 3 to 5 kV) of polarity opposite to that of the toners is applied to the recovering roller. As a result, a part of the toners will be left on the brush fibers and newly coming toners will adhere onto the remaining toners, whereby the brush fibers will be soiled with the toners firmly stuck to the brush fibers. Consequently, there may occur a failure of cleaning because of the reduced electrostatic attraction force between the brush fibers and the toners, or a normal cleaning effect may not be obtained because the toners having once adhered to the brush fibers will be transfered back to the surface of the photosensitive member again. To cause the toners to easily move onto the recovering roller, there is also proposed such a method that electric charges on the brush fibers are eliminated by a corona electrode before the brush fibers including the toners adhered thereto comes into contact with the recovering roller. But in this case, the corona electrode is apt to be soiled due to dispersion of the toners, whereby there arises the problem that a stable discharge can not be sustained and the cost is increased.
On this occasion, when employing conductive brush fibers in place of insulative brush fibers, the brush fibers can not hold the proper potential and the toners having adhered on the surface of the photosensitive member will not move onto the brush fibers, so that the desired cleaning effect can not be attained.